According to well established prior art design, rotary capping machines typically utilize a revolving turret with multiple spinning spindles arranged around the circumference of the turret. Each spindle carries an adjustable, magnetic capping head at its lower end and is rotationally driven by the rotation of the turret or an independent electric motor through a gear train so that all spindles turn at the same speed. Frequently, planetary gear trains are used in such arrangements where the rotational speed of the spindles varies proportionally with changes in the rotational speed of the turret.
During high volume production of bottles having screw cap closures, it is frequently necessary to alter the through put rate of the bottles, i.e., the number of threaded caps applied to the bottles over a given period of time, to account for changes or disturbances in the manufacturing and handling processes either up line or down line of the rotary capping machine.
As the capping machine applies threaded caps onto the necks of bottles, the final tightness of the caps is a product of two components. One component is contributed by the static torque setting of the clutch in the capping head, whereas the other component comprises the rotational interia of the components of the capping head that are in contact with the cap being applied. The former component is fixed with any one torque setting of the clutch, but the latter varies with the rotational velocity of the spindle. The torque applied by this arrangement to the caps is therefore the sum of the two components and is variable with the speed of the capping machine. Thus, while the static torque setting of the clutch and the capping head will remain constant for any speed of the spindles, the rotational interia (commonly referred to "dynamic torque") will increase with an increase in the rotating speed of the spindles, and conversely will decrease with a decrease in the rotating speed of the spindles. Hence, when the turret speed is increased to yield a higher through put of bottles, this dynamic torque factor will likely overtighten the bottle caps, possibly causing an improper seal or breakage. Conversely, when the turret drive motor is slowed considerably below normal values, the dynamic torque factor will contribute very little to final cap tightness and as a result may not properly seat the caps onto their bottles.
U.S. Pat. No. 4,608,805 to Kelley, issued Sep. 2, 1986, discloses a rotary capping machine having a rotating turret and a plurality of spindles. A first variable speed motor drives rotation of the turret, whereas a second variable speed motor drives rotation of the spindles independent of the turret speed. Such independent control of the spindle speed must be controlled on an ongoing basis by an operator as there is no communication between the first and second variable speed motors.
Various other configurations have been proposed for controlling the final torque tightness of threaded caps, such as shown in U.S. Pat. No. 4,616,466, issued Oct. 14, 1986, and U.S. Pat. No. 4,535,583, issued Aug. 20, 1985, both assigned to the Shibuya Kogyo Company. However, these as well as various other such control mechanisms have proved inadequate to maintain cap tightness at high speed, i.e., high through put, operation and on a consistently reliable basis.